1. Field of the Invention
This invention relates to an anti-skid brake control system for a vehicle.
2. Description of the Prior Art
There has been known an anti-skid brake control system for a vehicle in which the slip ratio of each of the wheels is calculated and the braking force applied to the wheel is controlled so that the slip ratio of the wheel converges on a target slip ratio at which the friction coefficient between the wheel and the road surface is maximized in order to prevent the wheel from locking during braking.
The slip ratio of the wheel can be calculated by various methods. In one of the methods, the slip ratio of each wheels is calculated on the basis of the difference between the speed of the wheel and an estimated vehicle speed which is determined on the basis of the highest of the wheel speeds and the deceleration of the vehicle body.
The idea of calculating the vehicle speed on the basis of the highest wheel speed (the highest of the wheel speeds) is disclosed, for instance, in Japanese Patent Publication No. 57(1982)-7940.
However, when the anti-skid brake control is effected on the basis of the slip ratio calculated in the manner described above, there is a problem that the driver has a feeling that the braking effect abruptly deteriorates at extremely low vehicle speed intermediately before the vehicle stops.
That is, during braking, especially when the anti-skid brake control is being effected, the highest wheel speed fluctuates due to fluctuation in the brake fluid pressure, and accordingly, the estimated vehicle speed per se fluctuates.
Generally, during braking, though the difference between the vehicle speed and the wheel speed is large to some extent so long as the vehicle speed is large to some extent, the difference between the vehicle speed and the wheel speed is very small at extremely low vehicle speed intermediately before the vehicle stops.
As a result, at extremely low vehicle speed, the calculated slip ratio fluctuates by a large amount in response to fluctuation in the estimated vehicle speed, and the slip ratio alternately becomes larger and smaller, whereby unnecessary reduction of the brake fluid pressure is done immediately before the vehicle stops. This causes the driver to have a feeling that the braking effect abruptly deteriorates.